More than a decade ago, we reported that among dendritic cells, EpCAM was selectively expressed by epidermal Langerhans cells. In the past few years, we have revisited the utility of EpCAM as Langerhans cell surface marker and have extended our earlier studies and demonstrated that EpCAM expression, in conjunction with other surface markers, differentiates Langerhance cells from all other dendritic cells, including several recently described novel cutaneous dendritic cell subsets. In an effort to elucidate important aspects of EpCAM function in vivo, we identified a pre-existing targeted mouse embryonic stem cell generated by BayGenomics and, working with the CCR Mouse Knockout Core Facility headed by Dr. Lino Tessarollo, generated EpCAM +/- mice in which beta-galactosidase was inserted into one EpCAM allelle. EpCAM +/- mice were viable, fertile and indistinguishable from wild type littermates. Examination of beta-galactosidase expression as a surrogate for EpCAM revealed that EpCAM was expressed in a variety of developing epitehelial structures in skin and other organs. Mating of EpCAM +/- male and female mice gave rise to only wild type and heterozygous animals. No viable EpCAM-deficient pups were obtained. Assessment of embryos in timed pregnant females revealed that EpCAM-deficient embryos implanted and were indistinguishable from EpCAM-sufficient embryos until EGA 8.5 when they began to exhibit developmental delay. EpCAM-deficient embryos became nonviable and were resorbed soon thereafter. We subsequently determined that EpCAM was transiently expressed in conceptus-derived placentas with maximal expression at EGA 8.5-9.5. Detailed studies of placentas associated with EpCAM-deficient embryos revealed that they were small and thin with incompletely developed and poorly vascularized labyrinthine layers. EpCAM-deficient placentas also contained markedly decreased numbers of parietal trophoblast giant cells, a phenotype that has previously been associated with embryonic lethality. A manuscript describing these findings will be submitted for publication in the immediate future. Thus, although mechanistic aspects of EpCAM function remain to be elucidated, EpCAM clearly has one or more nonredundant roles in normal physiology. To gain additional insights into EpCAM function, we are in the process of generating mice with and EpCAM allele that can be conditionally deleted in a lineage-specific fashion. Working with Dr. Tessarollo and using recombineering, we have developed a targeting vector that will allow loxp sites to be inserted into the EpCAM locus. Ultimately, mice with targeted EpCAM alleles will be crossed with mice that express the recombinase cre in cells of various lineages to ask and answer relevant questions.